KUALA LUMPUR, Malaysia—There are macaques everywhere—climbing on the rocks, grooming one another as they sit on the forest floor. Others have babies on their backs as they trot along at a fair clip. The air is thick with humidity and it must be 35 degrees Celsius or more—the heavy gray clouds above look ready to crack into a noisy tropical thunderstorm at any moment. I'm making my way along a dense rainforest path with the noisy thrum of insects all around me.

I could be almost anywhere in the tropics, but here at Kepong, 16 kilometers north of Malaysia's capital Kuala Lumpur, the land holds something very special. For what seems to the untrained eye to be dense primary rainforest is in fact an area that was denuded as recently as the 1920s. Scrubby vegetation, made up of grasses, ferns and fast-growing pioneer bushes and trees, was all that remained after the forest had been stripped to allow tin mining and vegetable cultivation.

But in 1926 pioneering forestry scientists in the pay of the British colonial government started a grand experiment to reseed, and it's now the only place in the world where there is a very large tract of artificially seeded rainforest—one which has slowly been regenerating for nearly 90 years.

Today the Forest Research Institute Malaysia (FRIM) has a regeneration experiment that covers 500 hectares and—aside from the years of Japanese occupation during World War II—they have a largely unbroken record of the number, diversity and size of rainforest trees growing here.

What researchers are learning from this experiment is significant, as so much of Malaysia, Indonesia and other tropical nations have been deforested. If the United Nations's plans for reducing emissions from degradation and deforestation are successful, and deforestation can be slowed and eventually reversed, how long will it take—and how easy will it be—to replace the rainforest we've lost?

"This is certainly one of the oldest rainforest regeneration experiments around—if not the oldest—and it's really big in scale," says Bill Laurance, Australian laureate and expert on tropical forests at James Cook University in Cairns, Queensland. "That's important because it means you can limit spillover effects from nearby old-growth rainforest. These can create the misleading impression that more species survive in the regenerating forest than is actually the case."

Laurance says that reforestation experiments are ongoing in many parts of the tropical world: "in Costa Rica, Brazil, Panama, Puerto Rico, tropical Australia and other locales. Some are trying to promote regeneration of timber species, whereas others are trying to yield a broad array of ecosystem services such as carbon storage, maintenance of water flow and quality, and biodiversity."

These kind of experiments "tell us a lot about rebuilding a rainforest," he says, as well as inform us about "what we can do that will help forests recover their biodiversity, carbon storage and other ecological functions in as short a time as possible—and hopefully in a way that roughly approximates the forest that was there originally."

The plantation experiment set in motion by the British was intended to help make the best use of the colony of Malaya's (as Malaysia was known under British rule) timber resources and was also instructive in the development of rubber plantations that were spreading across the region to fuel the booming new automobile industry.

In the early 1900s Malaya's forests were known to be extraordinarily diverse, and the British wanted to know which of the timber trees would be best for cultivation and which could be used on land that had been previously deforested. After 1963, however, as Malaysia's forests have dwindled and learning how to manage this resource sustainably has gained importance, the institute has morphed into one that today is interested in ecology, biodiversity and sustainable forestry—and the experiment has become useful for another reason entirely.

Shamsudin Ibrahim, director of FRIM's Forestry and Environment Division, says that they have planted a limited number of tree species in the hope that nature will do the rest of the work itself. He says the experiment here has been a valuable lesson in what to plant and when, in how to space out the seedlings, and in what kinds of plants and animals will readily return by themselves.

In 1926 one area in the former mining site was planted with nurse trees that would provide shade and improve the soil before the planting of dipterocarp species (large, hardwood rainforest trees valuable as timber). The nurse tree species introduced included Pterocarpus peltocarpum and camphor (Dryobalanops aromatica), locally known as kapur, and heavy hardwood species (such as Neobalanocarpus heimii). "The same technique of introducing nurse trees prior to planting with valuable timber species was repeated at all degraded sites in FRIM in early 1927, and today the site is fully covered with many tree species, which to untrained eyes of many visitors appears to be a multilayer vegetation, with no distinct differentiation between planted and natural species," Ibrahim says.

Initially around 15 tree species were planted, including fast-, medium- and slow-growing timbers, but now the area has more than 70 species of trees, says FRIM's director general Abdul Latif Mohmod. "The timber species that were initially planted acted as nurse trees to provide a microenvironment conducive for other timber species to establish and grow with minimal maintenance." The forest today looks very similar to mature rainforest—it has much of the structure of established forest, but what it lacks is the enormous ecological diversity.

The researchers are confident that will come with time, although it could take up to 1,000 years to get the majority of the diversity of plant species back. "What is being demonstrated at FRIM is an excellent testimony that restoration of tropical forests is feasible, but the process of returning them to their primary condition will definitely go beyond 90 years," Mohmod says. "The ecosystem is recovering but has not reached its original state, because in undisturbed tropical lowland forests the species richness is estimated at more than 200 species per hectare. The trees may reach their maturity at 60 to 80 years, but the maturity of the ecosystem may extend to hundreds of years or longer."

And getting the tools and know-how we need to help rainforests return is an urgent task, Laurance says: "In many parts of the world—such as the Philippines, Madagascar and the Brazilian Atlantic forests—there's hardly any old-growth forest left. We need to learn how to help these imperiled forests recover faster and better."